In the prior art, a number of devices have been developed for providing information about the location of a beam of light and particularly a coherent beam of radiation generated by a laser. Information about laser beam position is important for a number of reasons. For example, when a laser delivery system is being assembled or adjusted, knowledge of the beam position is necessary to permit the alignment of the beam with respect to the delivery optics. Proper alignment is critical to the performance of the delivery system. Laser beam position information is also important in devices where the movement of the beam plays a part in making test measurements. For example, certain laser test devices cause a beam to be reflected off a sample. Changes in the position of the reflected beam can be used to gain information about various surface characteristics of the object.
The principal devices found in the prior art for detecting the position of laser beams are silicon based photodetectors. Silicon photodetectors are designed to generate a voltage based on the light that falls on the detector surface. A standard, single element photodetector can provide information about the intensity of the beam. Position of the beam within the surface of the detector requires that the detector surface be divided into sections. The standard devices for this purpose fall primarily into two classes; specifically bi-cell and quadrant photodetectors.
A bi-cell detector includes two detector elements separated by a central linear gap. When a laser beam is perfectly centered on a bi-cell detector, the output from both sides of the detector will be equal. If, however, the beam moves more to one side of the detector than the other, the output voltage from the two sides will be different. The voltage difference can be used to determine the position of the beam along one axis. Quadrant detectors are similar to bi-cell detectors except that the detector surface is broken into four radial quadrants each generating independent voltage information. The independent voltage levels from the four radial quadrants allows both the X and Y position of a laser beam to be derived.
Quadrant and bi-cell silicon photodetectors have been successfully used to measure low-power visible laser beams. Unfortunately, normal silicon based devices cannot be used for the long infrared wavelengths generated by, for example, CO.sub.2 lasers. This is a critical problem since the infrared beams are invisible to the human eye. Therefore, some method must be provided to permit alignment of these beams.
In the prior art there have been developed some cryogenically cooled detectors for indicating the position of a far infrared laser beam. Unfortunately, these devices are expensive and, like the silicon detectors, are only suitable for use with low power beams. Specifically, silicon photodetectors and the cryogenically cooled detectors operate in power ranges under a watt.
Today there are a large variety of commercially available lasers which generate laser beams far in excess of a watt. Ion lasers generate visible beams over 20 watts. Similar powers are generated by commercially available YAG lasers. Industrial CO.sub.2 lasers generate beams having power outputs measured in the kilowatt range. None of the position detecting sensors found in the prior art can handle these high power beams.
Since there are no suitable position detecting devices for high power infrared beams, designers of industrial lasers must resort to crude techniques for aligning the optical elements in a beam delivery system. For example, a thin piece of wood will be moved by hand into the general area of the beam path. The beam will burn the wood giving the technician information about the position of the beam. The delivery optic can then be approximately aligned based on this crude information. These steps are performed for each optic between the laser output and the final lens used to focus the beam on the workpiece. As can be appreciated, this approach is both dangerous and not particularly accurate. It would be desirable to provide a beam position detector which could be used to align beam delivery optics for high power laser systems.
In the late 1960's, as the power of commercial lasers was being increased, not only was there a lack of position detectors, but there was a lack of a good meter to provide a measurement of the output power of the beam. A significant amount of effort was expended to develop meters which would measure high power laser beams. One solution is described in U.S. Pat. No. 3,596,514 issued Aug. 3, 1971 to Mefferd, et al. and assigned to the assignee of the subject invention and incorporated herein by reference.
In the latter device, a thermally conductive planar disk is mounted in a structure for dissipating heat. The disk includes a plurality of thermocouples connected in series to define a thermopile. The thermopile is arranged in a circular array about a target area upon which the laser beam is focused. As the laser beam heats the disk, the thermopile will generate a voltage proportional to the power level of the beam. The voltage is supplied to a meter which is calibrated to display the power of the beam in watts.
The commercial embodiment of the power meter described in the latter patent has been successfully sold by the assignee herein for the last 20 years. One advantage of the thermocouple array is that the device is relatively insensitive to the position of the beam on the disk. More specifically, since the thermocouples are connected in series, the voltage generated represents the total heat absorbed by the disk, and therefore will be substantially independent of beam position.
The disks utilized in the commercial embodiment of the meter disclosed in U.S. Pat. No. 3,596,514 are covered with a highly absorptive black coating material. It has been found that after prolonged use with high-power laser beams, the surface coating will sometimes become bleached or otherwise affected. This degradation can create some difficulties in obtaining reproducible power measurements. For example, if an area on the detector is bleached to some degree, a visible beam focused on that area will be partially reflected. If part of the beam is reflected, less energy is absorbed than if the beam was focused on an undamaged area. Accordingly, if some bleaching has occurred, it is necessary to align the beam at the same spot on the detector to obtain consistent readings from one measurement to the next. However, positioning of the beam at the same spot by eye can be difficult with bright laser beams. Accordingly, customers who wanted to insure reproducible results have had to recoat the surface of the detector at periodic intervals to remove the bleaching effect. It would be desirable to develop a product which did not have to be recoated to avoid this problem.
Accordingly, it is an object of the subject invention to provide a power meter which can also give information about the location of the beam on the surface of the detector.
It is another object of the subject invention to provide an apparatus for generating information about the position of high power laser beam.
It is a further object of the subject invention to provide a new and improved power meter which can generate information both as to the power level of the beam and the beam position.
It is another object of the subject invention to provide an apparatus for locating the position of infrared laser beam.
It is still a further object of the subject invention to provide an apparatus which can generate a real time display indicating the position of a laser beam on a detector.
It is still another object of the subject invention to provide a power meter having a longer lifetime.
It is still a further object of the subject invention to provide a device capable of generating information about the position of a laser beam which functions over a wide range of wavelengths and beam powers.